1. Field
Aspects of the present disclosure relate generally to wireless communication systems, and more particularly, to an apparatus and method for improving the performance of a linear equalizer with multiple receive antennas.
2. Background
Wireless communication networks are widely deployed to provide various communication services such as telephony, video, data, messaging, broadcasts, and so on. Such networks, which are usually multiple access networks, support communications for multiple users by sharing the available network resources. One example of such a network is the UMTS Terrestrial Radio Access Network (UTRAN). The UTRAN is the radio access network (RAN) defined as a part of the Universal Mobile Telecommunications System (UMTS), a third generation (3G) mobile phone technology supported by the 3rd Generation Partnership Project (3GPP). The UMTS, which is the successor to Global System for Mobile Communications (GSM) technologies, currently supports various air interface standards, such as Wideband-Code Division Multiple Access (W-CDMA), Time Division-Code Division Multiple Access (TD-CDMA), and Time Division-Synchronous Code Division Multiple Access (TD-SCDMA). The UMTS also supports enhanced 3G data communications protocols, such as High Speed Packet Access (HSPA), which provides higher data transfer speeds and capacity to associated UMTS networks. High Speed Downlink Packet Access (HSDPA) is a data service offered on the downlink of W-CDMA networks.
The downlink performance of HSDPA capable user equipment (UE) can be improved significantly by employing a chip level equalizer to combat the frequency selective behavior of the wireless channel. The performance of an equalizer receiver can be improved even further by employing multiple receive antennas (Rx diversity or RxD) at the UE to exploit spatial diversity. A typical HSDPA equalizer architecture (with Rx diversity) comprises of two linear filters (one for each receive chain), the coefficients for which are computed based on the criterion of minimizing mean squared error between the transmitted and received chip level signals. The signals received at the two receive antennas are processed with independent automatic gain control (AGC) circuits and stored in two different sample buffers for further processing. The AGC circuits for the two receive chains are operated independently in order to minimize the impact of quantization noise on each Rx individually. The computation of the equalizer taps involves the inversion of the received signal covariance matrix. In order to make the inversion robust, the covariance matrix may be “conditioned” or “regularized” by augmenting its diagonal with a non-zero term. However, in the presence of independent AGC circuits, covariance matrix conditioning degrades equalizer performance in scenarios where the two receive antennas experience a significant power differential or imbalance (e.g. due to physical phenomena like channel fading and shadowing or even due to the user's grip on her phone, which may block one of the antennas).
Accordingly there is a need to improve performance of a linear equalizer in a receiver of a HSDPA enabled UE having multiple receive antennas.